The chemical and petrochemical industry produces worldwide thousands of compounds each year and is still in expansion. The global market of this sector is huge and only during the first 9 months of 2004, the sales in the USA summed up US $110.4 billion for 26 companies of the chemical sector.
Unfortunately, linked to the great economical benefits obtained from the chemical and petrochemical production, large volumes of wastewaters and gas emissions containing many different toxic and recalcitrant priority pollutants are being discharged (Razo-Flores et al. 2006). Priority pollutants refer to industrial chemicals, which have serious effects on the environment and on public health and thus, are listed by the Environmental Protection Agency of USA. Among the priority pollutants generated by different industrial sectors are compounds, such as azo dyes, nitroaromatics, chlorinated aliphatic compounds, chlorinated aromatics and metalloids, which remain unaffected in conventional aerobic wastewater treatment systems. However, under anaerobic conditions, these contaminants can undergo reductive transformations generally forming compounds, which are aerobically biodegradable (Field et al. 1995). During the last two decades, evidence accumulated showing the potential to convert electron-withdrawing pollutants in high-rate anaerobic bioreactors, such as upflow anaerobic sludge bed (UASB) and expanded granular sludge bed (EGSB) systems (Cervantes et al. 2001; dos Santos et al. 2005). However, reductive transformation of many different recalcitrant compounds proceeds very slowly due to electron transfer limitations and to toxicity effects leading to poor performance or even collapse of anaerobic bioreactors (van der Zee et al. 2001).
Quinones, redox active groups very abundant in humic substances, have been proved to accelerate the transfer of electrons during the reductive (bio)transformation of a wide variety of priority pollutants, increasing the reductive conversion rates by one to several orders of magnitude (Field & Cervantes 2005). Quinoid redox mediators not necessarily have to be supplied abundantly in anaerobic bioreactors to accelerate reductive transformation of electron-withdrawing contaminants, as they are being regenerated during the transfer of electron from an electron donor to the pollutants. Nevertheless, continuous addition of redox mediators should be supplied in anaerobic bioreactors in order to achieve increased conversion rates, which raise the costs of treatment.
An approach to eliminate the prerequisite of continuous supply of redox mediators is to create a niche for their immobilization in anaerobic reactors. However, scarce attempts to apply immobilized quinoid redox mediators for the anaerobic reduction of electron-withdrawing pollutants have been reported. One of the alternatives available considers the application of activated carbon as a natural source of quinoid redox mediators and its potential for reducing azo dyes have been explored in anaerobic bioreactors (van der Zee et al. 2003). However, the catalytic effects of activated carbon gradually decrease attributed to its long-term wash-out from the reactor. Moreover, the redox active groups in activated carbon have a redox potential, which can effectively transfer electrons to a limited number of pollutants. Another immobilizing approach reported is to insert quinoid redox mediators within different materials through polymerization procedures (Guo et al. 2007). Through this immobilizing technique, quinoid redox mediators remain entrapped within the synthesized polymer. Disadvantages of this strategy are: 1) mass transfer limitations since a major part of the redox mediators are embedded within the polymeric material making its accessibility dependent on diffusion; 2) gradual lost of redox catalysts due to disruption of the polymeric material owing to weak mechanical strength of the materials explored so far.
No attempts to immobilize quinones on exchange resins have been reported and a number of advantages can be underlined with this strategy. By properly selecting an ion exchange resin and a quinoid redox mediator, two major goals can be achieved: 1) stable attraction between active functional groups in ion exchange resins and the redox mediator; and 2) redox active groups (e.g. quinones) remain available for catalysis, because other functional groups, with a greater attraction for the exchange resin, can link both materials. Furthermore, a redox mediator with the proper structure and redox potential to effectively transfer electrons from an external electron donor to a specific electron-withdrawing pollutant could be selected. The last observation is particularly important considering that redox properties greatly differ among the distinct catalysts available in the market, resulting in different impacts towards the conversion of a particular pollutant (Field & Cervantes 2005). Moreover, quinoid redox mediators could be carefully immobilized on the surface of non-porous ion exchange resin particles decreasing mass transfer limitations during catalysis. Additional benefits of considering ion exchange resins is that several of these polymeric materials have the proper mechanical strength to prevail unaffected in high-rate anaerobic reactors and a specific weight, which prevents their wash-out during continuous operation of anaerobic bioreactors.
There are several advantages of considering quinoid redox mediators for reductive transformation of electron-withdrawing contaminants. Quinones are very abundant in humus, which is the most plentiful organic fraction accumulating in terrestrial and aquatic environments. Therefore, humic substances represent a plentiful and cost-effective source of quinoid redox mediators. Moreover, humic substances have a remarkable stability in the environment. Indeed, high molecular weight humic materials have a residence time longer than 500 year (Stevenson 1994).